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FUNDAMENTAL CHEMISTRY AND CONTROL OF STRUVITE PRECIPITATION James Doyle School of Water Sciences, Cranfield University England. Struvite Formation. Introduction. Struvite forms according to the general reaction shown below: Mg 2+ + NH 4 + + PO 4 3- = MgNH 4 PO 4 .6(H 2 0)
School of Water Sciences, Cranfield University
Struvite forms according to the general reaction shown below:
Mg2+ + NH4+ + PO43- = MgNH4PO4.6(H20)
However this equation is a simplification of the chemistry involved in struvite precipitation. Struvite precipitation is controlled by pH, degree of supersaturation, temperature and the presence of other ions such as calcium and can occur when the concentrations of magnesium, ammonium and phosphate ions exceed the solubility product (often denoted as Ksp) for struvite. The relationship between Ksp and pH indicates that struvite solubility decreases with increasing pH, which in turn leads to an increase in the struvite precipitation potential of a solution.
Struvite (MgNH4PO4.6H2O) formation in wastewater treatment has gained greater consideration with the introduction of the EEC Urban Waste Water Treatment Directive (UWWTD, (91/271/EEC). The changes in legislation regarding the removal of nitrogen and phosphorus from wastewaters has led to operational problems particularly when treating the sludges derived from Biological Nutrient Removal (BNR) processes.
Sludges wasted from BNR if anaerobically digested will re-hydrolyse poly-phosphates previously formed in the aerobic treatment stage, magnesium and phosphate ions will also be released. Since ammonium ions will be high in concentration struvite precipitation can occur.
Struvite crystals formed from a real
150mm diameter pipe reduced to 60mm after 12 weeks by struvite precipitation.
Impact of pH upon the pKsp value and the Ion Activity Product/Ksp ratio.
A computer model predicting struvite precipitation potential (SPP) was compared to struvite precipitation in real and synthetic liquors. The pH values ranged from 6.5 to 8.8 with masses of struvite formed ranging from 0mg l-1 at pH values below 7.5 to values exceeding 350mg l-1 at pH values above 8.8. Data from the jar tests was compared with a crystalline deposit taken from Coleshill SDP operated by Severn Trent Water plc. X-ray diffraction (XRD) and dissolution experiments were used to identify the purity of struvite precipitates formed in both real systems and the simulated precipitation experiments using jar tests.
The precipitates recovered weretested for magnesium, phosphorus and calcium concentrations via dissolution experiments. The initial concentrations of magnesium and phosphorus having been measured prior to struvite precipitation were then compared to the concentrations of magnesium and phosphorus remaining in solution following precipitation. From these data the mass of struvite formed can be calculated assuming that a molar removal of magnesium equates to the precipitation of a mole of struvite. The concentrations of magnesium and phosphorus in the precipitates were calculated as weight percentages of the theoretical weight percentages for struvite.
Mass of struvite precipitated and the precipitation potential calculated by a model with increasing pH.
The percentage of the theoretical concentration of phosphorus and magnesium measured following dissolution of various masses of struvite.
Struvite precipitation is predominantly governed by pH.
Application of models can determine a solutions’ struvite precipitation potential.
Manipulation of solution chemistry can increase or decrease
struvite precipitation potential.
Struvite precipitation can be influenced by other ions in solution e.g. Calcium ions.
Research into the kinetics of struvite precipitation has been initiated. Using modified impellers to which different materials may be attached, the kinetics of struvite precipitation has been investigated. Struvite formation upon stainless steel, teflon and acylic has generated data suggesting that surface roughness has a profound impact upon struvite fouling. This data can be used to assess materials for use in struvite reactors.
Stainless steel impeller blades before and after experiments to assess the kinetics of struvite precipitation.
The authors would like to express appreciation for the support of the sponsors: EPSRC and Severn Trent Water Ltd